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Project Help and Ideas » using a strain gauge to build shock dyno

January 11, 2011
by microdad17
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I want to build a shock dyno to test racecar shocks and I need help. I have little to no knowledge of electronics or programming. I purchased the Nerdkit because I found the straingauge tutorial on Youtube. I have had this idea for several years and thought this may be the answer. I am willing to try and learn the electronics part of this project but have no idea of what needs to be done. Let me layout the project as I have it in mind for now.

  1. Push down button ( using rocker switch). Fire air solenoid to open valve to air solenoid and extend air cylinder (this puts compression on shock). a. This will put “tension” on the strain gauge. It will give a negative voltage signal. b. I need to record and display the highest negative voltage in lbs on the lcd screen and have it labeled compression.

  2. Push up button ( using rocker switch). Fire air solenoid to open valve to air solenoid and retract air cylinder (this puts rebound on shock). a. This will put “compression” on the strain gauge. It will give a positive voltage signal. b. I need to record and display the highest positive voltage in lbs on the lcd screen and have it labeled rebound.

Like I stated above, I know basically nothing about electronics. But I am willing to TRY and learn. As far as the programming part of the project, well it may as well be in a different language to me. And I am really not that interested in learning that part (I am willing to pay to have that part done). This is the starting point of my dyno. I would like to have it go further and be more automated but for now this will work for me. Please keep in mind that I will need step-by-step instructions.

My questions are: 1. Is this something that can be done with the nerdkit? Or do I need to be looking in another direction? 2. If this is something that the nerdkit can do, is there anyone willing to help me with this project?

Thank you Paul (microdad17)

January 12, 2011
by Ralphxyz
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A big problem I came across with my strain gage project using the strain gages from a bathroom scale was changing linear motion (tension) to compression. The bathroom strain gage only work in compression. You could purchase a "Load Cell" and be able to measure "tension" but that will cost you possible $500.00 -$1,000.00 or more.

You can use a lever to get the compression but you would need lots of room.

I have a "idea" for changing tension to compression in a limited area but so far it is just a idea.

So compression is easy to handle you can just use the strain gage tutorial project and the tempsensor project for output to the LCD. It's the tension (linear motion) that is hard to do (cheaply).

Can you post a drawing of how you are picturing your setup?

Ralph

January 13, 2011
by microdad17
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Shock dyno setup

January 13, 2011
by microdad17
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Thank you for your reply Ralph. I am not sure I worded that properly before. When I said negative voltage what I meant was, a drop in votage. I have a straingauge from a bathroom scale that is hooked up to a Wheatstone Bridge. The Wheatstone Bridge has a "pot" built in. (this is all on a bread board at this point. Because I am not sure I have it built correctly.) Using the "pot" I can change the voltage signal and zero it to a nominal number on the volt meter. Using force on the strain gauge I can change the voltage +- from this nominal number. On the volt meter I can zero the voltage to lets say 1.500 volts. I can vary this voltage to 1.505 pushing straingauge one direction, and 1.495 pushing straingauge the other direction. I think I need more resolution to get a good reading. I am not sure what to do with that. But this showes that the straingauge does work in both tension and compression.

any help or suggestions are greatly appreciated.

thank you Paul

January 13, 2011
by Ralphxyz
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If you are getting a voltage reading you can use the Nerdkikit temperature sensor project to display values on the LCD.

I sure like to see how you have your strain gage setup to get tension.

I actually Googled Shock Dyno to see what the heck you were talking about.

You have a very clean setup there I like using the air cylinder, will that cycle fast enough?

Is that a double action air cylinder or does it have a spring return?

What program are you running on your laptop to get the graph?

Ralph

January 13, 2011
by microdad17
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I would appreciate it if you could tell me how to hook my straingauge up to the temp sensor setup.

The air cylinder is double acting. I have a dyno now that uses this type of cylinder. It has a magnetic piston. I have reed switches mounted on the outer housing. All this does is start and stop a timer. The times are then put into an Open Office template for the graph.

There are a couple types of tests that can be done on the shocks. 1. is a 20 point test. Air pressure settings from 20-100psi in 5lb increments 2. is a 5 point test. air pressure settings from 20-100psi in 20 lb increments. This is the test I use on a daily basis.

I would like to have a better dyno for testing my shocks. I need one that puts out data in lbs. Not time. This is the reason for the project. Getting the data is step 1 of the process. If we(all of you and myself) can get this working to get me atleast one number for now, I would be very happy. Atleast then I will know this will work and I can continue to pursue this.

January 14, 2011
by Ralphxyz
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I used a op-amp to boost the signal but if you are already getting voltage just connect your + to PIN 23 (PC0) in place of the LM34.

If you need to boost the signal you can use a op-amp. Using a common ground for your strain gage and Nerdkit.

Just do the tempsensor project and replace the tempsenor. It will read in degres but for now you just want to see a reading.

You can easily change the readout to pounds or what ever.

Ralph

January 15, 2011
by keithka
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Do you have a plan for measuring the damper displacement? Probably some variation of the digital caliper project would work. I used to be an engineer for a pro Formula Mazda team, and worked as the data guy for an ALMS (P2) team, so I have thought quite a bit about how fun it would be to make a shock dyno someday. I also recommend starting from the temp sensor project, partly because you'll also want to log damper temperature. Damper temperature has a huge effect since the oil in the damper gets a lot less viscous as it heats up, and the damper does build a fair amount of heat during a session - especially if it's a formula car where the rear dampers sit on the gearbox.

It seems like if you have two sensors sandwiched into a rigid U-shaped bracket , you can measure rebound force with one, and compression with the other. Let me try some ascii art:

----------
| rebound force sensor
| connection to damper end
| compression force sensor
----------
    |
 air cylinder attached to U-shaped bracket
January 16, 2011
by microdad17
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pic 1 pic 2 pic 3

Sorry for the blurry images

January 16, 2011
by microdad17
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The strain gauge that I have gives a voltage reading in both directions(compression and rebound). So I only plan on using 1 for now. I have the tempsensor project completed. It works fine. I removed the LM34 and plugged in the + from the amplifed straingauge. That does not work either.I dont know if it is noise, or what. If you unplug your LM34 all you get is numbers jumping around. That is all I get. I have no idea what to do here.

If I understand your question about the damper displacement, The air cylinder will give a know force. I can use the known force vs lbs on the straingauge to calculate the graph. As far as temperature,you are correct. The shocks work differant the hotter they get. Right now I use a hand held infrared unit.

January 17, 2011
by Ralphxyz
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Do you still have the ADC: value displayed on the LCD?

Remove the strain gage lead from Pin 23 and jump Pin 23 to ground (-) do you have a steady zero reading?

Jump Pin 23 to Vcc (+) do you have a solid 1023 (1024)?

What is the voltage you are getting on your strain gage circuit?

If you have 0 - 5 volts from your strain gage it seems like it would show up.

You are correct if Pin 23 is allowed to float I get between 253 to 262 ADC.

Ralph

January 17, 2011
by microdad17
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"Remove the strain gage lead from Pin 23 and jump Pin 23 to ground (-) do you have a steady zero reading?"

Yes that is correct. zero reading

"Jump Pin 23 to Vcc (+) do you have a solid 1023 (1024)?"

Yes this is also correct.

"What is the voltage you are getting on your strain gage circuit?"

voltage is 1.590zero load. I can change this between 1.586 to 1.594 using the straingauge and a screw driver. Just applying pressure to both side of the straingauge.

When I plug the wheatstone bridge into pin 23 on the Nerdkit, it acts the same as if I just unplugged the LM34.

I have to be missing something. I just dont know what. The amp that I am using is from radio shack. I dont really think that is the problem since it seems to work as it should.

Maybe its the code that was used to program the mcu. Is there a way you can email me the files(all of them including the makefile) for the straingauge? paul@a1machine.net

Frustration is setting in on this project. I have been working on this project for around 2 years. I have even had it to the point of the Wheatstone bridge. And that worked. I had no way to read the voltage other than a multimeter. That wont quite get me what I need. I thought when I found the tutorial on the bathroom scale on Yahoo, that I had found the answer I was looking for.

Please help. thanks Paul

January 18, 2011
by Ralphxyz
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I used four strain gages making up the wheatstone bridge.

I do not know if the problem comes from using just one strain gage, I believe you would have to have critically matched resistors when using just one strain gage.

If you are getting 1.5v or so that should show up as ADC: 313 of 1024 or there abouts.

I use a 100k pot with the wiper to pin 23 to simulate a variable input for testing.

I am just using the Nerdkit tempsensor project code unmodified so if you have the tempsensor project running then it seems like it is something to do with your strain gage amplifier setup.

You do have a common ground with the mcu and strain gage?

Can you post a schematic of your wiring?

I and others have had the mcu and strain gage working together so you will get it working soon also.

Ralph

January 18, 2011
by Jalex
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Hi I saw strain gages at $14 - $15 at jameco.com and many other great sensors too if this is of any help.

January 19, 2011
by Ralphxyz
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Jalex you should post a link and not just make a open reference.

I searched jameco.com for strain gage and never found any. I found load cells and lots of other interesting sensors but no strain gages.

Ralph

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